The present application is related to a co-pending patent application, entitled "Method and Apparatus for Starting Difficult to Start Electrodeless Lamps," Ser. No. 08/696,706, which is hereby incorporated by reference.
Electrodeless lamps with quiescent fill pressures in excess of one atmosphere of pressure and/or electronegative fill constituents are typically difficult to start. Both high pressure and electronegative fill additives provide mechanisms that suppress the growth of the free electron population. A certain local density of these free electrons is crucial for a successful bulb ignition event. In order to overcome the ignition inhibition mechanisms, one typically increases the amplitude of whatever mode of excitation that one is using. In electrodeless lamps, the pitfall in this approach appears when the electric field strength required to ionize the bulb fill exceeds the Paschen parameter of the air and structure surrounding the bulb. In this case, all microwave or RF energy is delivered to an arc, which prevents the bulb from igniting and can destroy the lamp.
U.S. Pat. No. 5,571,439 discloses a variable power supply for a magnetron used in heating applications. The power supply includes two identical power units supplied from a three-phase AC main line. Each power unit is connected to a magnetron whose microwave output is coupled to an ultraviolet lamp through a wave-guide connected to a microwave cavity in which the lamp is secured. The lamp is used for curing purposes, such as in a printing application. In an electrodeless lamp, the microwave is coupled to a discharge forming fill disposed inside an envelope. The envelope contains a field or secondary electron emission substance disposed on a given region of the inside wall of the envelope. An exterior high voltage electrode is brought in contact with the envelope adjacent the given region containing the starting substance. The power sources and the electrode are energized to start the lamp. The electrode is then withdrawn away from the envelope after starting.
Applicants found that starting a hard to start lamp using the prior art power supply had been unreliable. At times, the lamp would start on a first attempt. At other times, several attempts would be required before the lamp would start. Applicants found that the high voltage impulse delivered to the field or secondary emission substance must be reliably applied near the peak of the magnetron current. Further, both magnetrons must be phased so that a period of high electric field is created of sufficient duration that the free electron population achieves the requisite local density for ignition. The extant state of the art power supplies did not satisfy these requirements resulting in unreliable lamp starting.